The present invention relates to a zoom lens and particularly to an optical system suitable to a so-called telephoto type zoom lens having an angle of view at the wide angle end which is smaller than 40.degree..
If the focal length at the telescopic end is quite long and the diameter is large, a lens diameter (front lens diameter) at the front end of the object side will be considerably large. Therefore, when a conventionally known focusing system in which an entire focusing group is shifted is employed, the weight of the total focusing lens will become very heavy, and a warp will be generated in the lens barrel while the lens is shifted. Thus, deterioration of properties will occur by eccentricity of optic axis, and the barrel wall thickness of the lens barrel must be thick for preventing the same. Accordingly, the total system becomes large and heavy, and its handling characteristics become poor.
On the other hand, a focusing lens having a number of lenses reduced is known in U.S. Pat. Nos. 3,594,066 and 3,598,476. Such zoom lens is suitable to a wide angle photographing.